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Analysis The Coalition government, as part of its ongoing strategic review of UK defence, may decide to postpone replacement of the Trident nuclear deterrent by five years. What would that mean?

Immediately in practical terms it would mean that spending on the replacement systems would be pushed mostly back out of the ten-year projections. As it has been made clear that the replacement is to be paid for from the regular Defence budget, this would mean some billions of pounds freed for other MoD areas.

However, there's a reason why things should be replaced or renewed at the end of their planned life: if you keep on operating them, running costs start to climb. Five years of running life-expired Trident boats will cost more - a lot more - than five years of running new ones. Then, as defence kit in general inflates in price faster than normal things do, delaying five years will also make the replacements significantly more costly in real terms than they would have been.

Bottom line, looking back in 20 years' time with a new like-for-like replacement of Trident in service, analysts would probably assess that the Coalition's five-year delay back in 2010 had cost the British taxpayer several billion pounds compared to going ahead in this decade - as both Labour and the Tories promised the voters they would.

There is, of course, another possibility - that there might be no like-for-like Trident replacement. One obvious plan is that instead of Trident intercontinental ballistic missiles (ICBMs) and the necessary special submarines to carry them, the UK might use cruise missiles shot from the torpedo tubes of relatively ordinary nuclear-powered attack submarines such as the Astute class now being delivered. The US-made Tomahawk, already in service on Royal Navy attack boats with conventional warheads, would be the obvious choice - it was originally developed to carry nukes, in fact.

This would be hugely cheaper than ICBMs and vertical-launch subs to carry them. This is because a cruise missile is actually just a robotic jet aeroplane, whereas an ICBM is a suborbital space rocket.

Unfortunately, a robot jetplane flying along at normal aeroplane altitudes and subsonic speeds can be shot down relatively easily. An enemy nation with a capable modern air-defence system will be able to do so - the prospect of retaliation by nuclear-tipped cruise missiles will not deter such a nation from mounting strikes against the UK if it really wants to. (And if it keeps some of its own ICBMs in reserve, it can still deter our allies from avenging us.)

By contrast, an ICBM-launched warhead falling out of space at many times the speed of sound is effectively unstoppable. Even the mighty USA, after decades of effort and scores of billions spent on Star Wars and missile defence, cannot reliably defeat ballistic weapons hurtling far above the atmosphere. No other nation has even the ghost of a chance of doing so.

That's why everyone wants ICBMs. They are a much, much more effective deterrent than cruise missiles or nukes delivered by manned aircraft. That's why North Korea and Iran and uncle Tom Cobbleigh work so hard to build "space rockets" - because if you can build an orbital launcher you can build an ICBM, and drop things out of the sky anywhere on Earth that you choose to do so.

You get what you pay for, with nuclear deterrents. This is why the last Labour and Tory governments, both elected on a platform that included an effective deterrent, have stuck to the gold standard of submarine ballistics. (The nuclear-powered submarine is also vital, as opposed to a land silo, as it is the only way of being sure that nobody knows where your missiles are. Thus an enemy cannot launch a strike at them, hoping that you will not be able to identify who is attacking and get your own missiles away before his arrive - perhaps in less than twenty minutes.)